Significant efforts have been made to formulate certain stainless steel alloys, such as martensitic precipitation hardening stainless steel alloys that exhibit superior properties for use in high performance articles. The potential for excellent strength-to-weight ratios, toughness, corrosion resistance, and stress corrosion cracking (SCC) resistance of articles formed from these alloys make them particularly well suited for use as aerospace structural components, such as flap tracks, actuators, engine mounts, and landing gear hardware. These properties, along with various manufacturing considerations, are strongly influenced by alloy composition, structure, heat treatment, and level of process control in the alloy systems. To obtain the properties necessary for high performance steel applications, careful and strict control of the alloying components, and the amounts and ratios of each, is generally required. Even slight adjustments in the alloying components or their amounts can significantly affect the properties and performance of these stainless steel alloys.
For example, early forms of martensitic stainless steel alloys employed copper as the major hardening element. These early forms of steel alloys are recognized as having good corrosion and SCC resistance, but have been found to have relatively low yield strength (YS<180 ksi). Because of the relatively inferior strength properties exhibited by martensitic stainless steel alloys employing copper, copper has not been favored as a major strengthening element in high strength stainless steel alloys.
Other martensitic stainless steel alloys have been developed that employ various amounts of aluminum as strengthening elements. These alloys can provide a yield strength greater than 200 ksi in the H950 condition (i.e., aged at an aging temperature of 950° F.) along with good ductility and toughness. However, the strength of this type of martensitic steel is still relatively low for many high strength applications. Other martensitic stainless steel alloys have been developed that employ both aluminum and copper as strengthening elements. These alloys exhibit much higher strengths (YS≧235 ksi), but fail to achieve acceptable levels of fracture toughness (K1C<65 ksi·in1/2).
Other approaches to forming martensitic stainless steel alloys involve the addition of titanium as the major strengthening element along with various amounts of copper, as the secondary strengthener, and proper nickel-chromium equivalents. These approaches provide relatively high strength (YS>240 ksi) and good corrosion resistance, but low toughness (Charpy V-notch (CVN)<10 ft/lb and K1C<65 ksi·in1/2). Other more recent developments include the addition of relatively high amounts of titanium (1.5%-1.8% by weight) and nickel that achieves high toughness, but at the possible expense of corrosion resistance and SCC resistance, due to the nickel/chromium imbalance. These latter alloying systems include a costly and time consuming cryogenic treatment step after solution heat treatment in order to achieve their high performance properties.
Still other high strength martensitic steel alloys employ a combination of aluminum and titanium as strengthening agents. These approaches can be divided into two groups: 1) alloys that employ relatively low amounts of aluminum and titanium and provide steels that exhibit relatively high toughness; and 2) alloys that employ relatively higher amounts of aluminum and titanium and provide steels that exhibit relatively high strength. However, it has been found that steel alloys that exhibit high strength generally exhibit low toughness, with Charpy impact energies being measured at only a few foot-pounds and facture toughness being less than 60 ksi·in1/2 at room temperature.
Other approaches to providing high strength steel alloys employ the use of one or more of silicon, beryllium and molybdenum as hardening elements to form steel alloys that exhibit very high strength, but low toughness. Because of their low toughness properties, these steel alloys are unsuitable for high performance structural applications.
Accordingly, further improvements would be a welcome addition to the prior art processes, which appear to lack well-established alloy design principles to determine which precipitation hardening elements should be used, how to combine precipitation hardening elements with other components of the alloy, and how the matrix chemistry should be correspondingly adjusted, to improve the characteristics of the stainless steel alloys. In particular, there is a continued need for approaches to increase the strength and toughness of martensitic stainless steel to provide greater integrity and performance in the articles formed therefrom.